In a radio communication system, spectrum is often reused in geographically distinct, but nearby, regions. In the case of two-way messaging systems, for example, inbound frequencies used by portable messaging units to transmit to the fixed equipment may be reused in nearby or even overlapping zones. Under certain reception conditions co-channel signals can travel further than normal and can be received in reuse zones for which the signals are not intended. In addition, transmitter spurious emissions may sometimes cause unintended "ghost" transmissions on nearby channels. These ghost transmissions will appear as low energy copies, often with sufficient signal strength to be decoded as error free signals.
Either of the two above described conditions can produce signals in one region which are not intended for that region. In prior art systems, such signals have caused some degree of confusion, and at the very least have resulted in unnecessary processing to distinguish the traffic intended for the region from that intended for another. A trivial solution to this problem would be to add bits to the transmission data streams used by the portable messaging units for encoding a destination field in each transmission. The overhead of transmitting such an additional field, however, is undesirable. Moreover, for the case of a protocol that is already in wide use, adding a new field is not feasible if unused bits are not available in the defined data streams for adding the field to the protocol.
Thus, what is needed is a method and apparatus that can distinguish a signal intended for a region from a signal intended for another region. The method and apparatus will preferably not require an additional field to be added to the data stream. More broadly, what is needed is a method and apparatus in a data communication system for communicating selected additional data without increasing the size of the data stream.